conduit: plugin constructor

[shiqizng]

Summary

refactoring plugin constructor to reduce boilerplate code

Test Plan

update unit tests

[codecov]

Codecov Report

Merging #1298 (1dc2ca7) into conduit (bd5a482) will decrease coverage by 0.14%.
The diff coverage is 21.21%.

@@             Coverage Diff             @@
##           conduit    #1298      +/-   ##
===========================================
- Coverage    58.82%   58.67%   -0.15%     
===========================================
  Files           78       78              
  Lines        10490    10496       +6     
===========================================
- Hits          6171     6159      -12     
- Misses        3786     3796      +10     
- Partials       533      541       +8     

Impacted Files	Coverage Δ
exporters/example/example_exporter.go	65.00% <0.00%> (-15.00%)	⬇️
exporters/filewriter/file_exporter.go	77.77% <0.00%> (-5.56%)	⬇️
exporters/noop/noop_exporter.go	58.33% <0.00%> (-12.51%)	⬇️
exporters/postgresql/postgresql_exporter.go	76.74% <0.00%> (-3.49%)	⬇️
importers/algod/algod_importer.go	78.16% <0.00%> (-1.15%)	⬇️
importers/filereader/filereader.go	84.37% <0.00%> (-1.57%)	⬇️
importers/importer_factory.go	21.42% <0.00%> (-3.58%)	⬇️
processors/blockprocessor/block_processor.go	64.22% <0.00%> (-0.44%)	⬇️
processors/noop/noop_processor.go	7.14% <0.00%> (-7.15%)	⬇️
exporters/exporter_factory.go	100.00% <100.00%> (ø)
... and 2 more

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[winder]

Could you move the handler func's next to the constructor functions in the *_factory.go files?

Other than this, the changes look good.